Monthly Archives: August 2016

Turbo Selection

K04 Summer Boost Data

With the BW K04’s back on the car I took some readings of boost pressure when going WOT at various engine speeds.  The data is being collected on an 85 degF day to compare with data I collected during colder ambient temperatures, along with allowing me to compare the variability of the K04’s to that of the TTE550’s and FT21’s.

BW K04 Boost Rise - Summer 85 degF
BW K04 Boost Rise – Summer 85 degF

The results show that the K04’s boost onset is slowed some with the warmer ambient temperatures.  Even though the K04’s are now boosting a little slower they are still more responsive than the TTE550’s and FT21’s were in colder temperatures.

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I also made a validation run with the K04’s to double check the exhaust manifold back-pressure reading.  The first time I made these readings I was a bit ‘relaxed’ about the process and didn’t think much of the data that was produced since I didn’t have anything to compare it against.

After taking the same reading with the FT21 and TTE550 and seeing that they both showed appreciably higher exhaust manifold pressure, for a similar level of intake manifold pressure, I began to question if I had correctly assembled the measurement apparatus when I recorded the K04 data.  Since the only way to be sure was to install the K04’s again, I had to wait a little while and then pull the engine out to put the K04’s back so I could record data to confirm if the initial round of data logging had been done properly or not.

BorgWarner RS4 K04 Intake Manifold vs Exhaust Manifold Pressure
BorgWarner RS4 K04 Intake Manifold vs Exhaust Manifold Pressure

Looking at the readings that were taken today, B3 & E3, I was relieved to see that they matched well to the readings taken several months ago.  Although it meant I did an engine pull for minimal benefit, the consistent results are satisfying.

Here’s how the three sets of turbo’s compare:

Turbocharger Intake Manifold vs Exhaust Manifold (Pre-Turbine Housing) Pressures
Turbocharger Intake Manifold vs Exhaust Manifold (Pre-Turbine Housing) Pressures

My belief is that this chart represents one of the keys for improving the performance of K04 alternative turbochargers.

BorgWarner RS4 K04’s cause a fair amount LESS back-pressure in the exhaust manifold as compared to two popular hybrid turbochargers.  Less back-pressure should improve the engine volumetric efficiency.

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I had a couple of requests to display the pressure data above in a different format.  Below are charts showing back pressure versus mass air flow, and the ratio of exhaust back pressure to intake manifold pressure versus engine speed.

F21 & K04 Pre-Turbo Exhaust Back Pressure vs MAF F21 - 42 degF / K04 - 75 degF Ambient Temperatures
F21 & K04 Pre-Turbo Exhaust Back Pressure vs MAF
F21 – 42 degF / K04 – 75 degF Ambient Temperatures
F21 and K04 Exhaust / Intake Pressure Ratio versus Engine Speed
F21 and K04 Exhaust / Intake Pressure Ratio versus Engine Speed
Upgrade Work

SRM Shroud Fitting

I’ve had a set of SRM IC shrouds sitting in the basement for quite a while waiting for some attention.  I have not used them because they don’t fit the SRM SMIC’s in their current state.  I knew it was going to take a visit from Mr. Dremel to begin hoping to make them fit – and today I finally decided to try and get started on this undertaking.

SRM Driver Side IC Shroud
SRM Driver Side IC Shroud

The primary problem with the driver’s side shroud is that the right portion of the shroud extended about an inch out past the mounting tabs, and also beyond the core itself.  This meant there was going to be a 1 inch gap along the right side of the core, and failing to secure the shroud to the core there was likely to leave additional large gaps elsewhere.

The best option I see is to cut the shroud in half, and then use rivets, or nuts & bolts, along with some epoxy adhesive, to reconnect the two halves in an overlapping manner that should eliminate the majority of the perimeter gaps.

The passenger side has an additional problem in addition to poor fitment.

SRM SMIC Passenger Side Shroud
SRM SMIC Passenger Side Shroud

The cutout for the tow hook was not present, it was a solid piece.  In the picture above the tow hook can be seen because I cut an opening for it to pass through.

On this side as well the shroud extends out past the mounting tabs leaving a gap along the edge of the IC core.  I’m going to cut this shroud in half similar to the driver’s side, but with a good bit more curvature along the lower half of the shroud reconnecting these two halves in a clean manner is likely to prove challenging.

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On a more positive note, the S4 is back up running again and after a couple of brief periods having the engine running I have not detected any leaks.  My next endeavor will be to gather some K04 spool-up data in warmer temperature conditions and to log pre-turbine exhaust gas pressure.

Testing

605 vs K04

Recent discussion of K04 vs larger (i.e. 605) turbochargers prompted me to pull up some data I’ve logged, 2-11 time for several turbochargers, and dyno plots from a number of cars with these turbo’s.

The 605 2-11 time data is from my S4, which utilized the 605.1 turbochargers, on stock S4 heads.  It spanned a number of months, and thus encompassed temperatures ranging from a low of 33 degF to a high of about 95 degF.  The spread in the 605 data is indicative of the affect that ambient temperatures had on these turbochargers. There is a similar spread with other turbochargers, but in the low airflow (engine speed) regime the 605’s appear to be more sensitive than the smaller turbochargers.

2-11 Time: Big, Medium, and Small Turbochargers
2-11 Time: Big, Medium, and Small Turbochargers

It is apparent that all three turbochargers generate boost pressure more rapidly as the engine speed (airflow) increases.

Another aspect of relative performance that I looked at was Torque Drop Off.  One of the claimed benefits of larger turbochargers is that the torque does not “fall off” like the smaller K04 turbo’s.  To assess how much less drop off there is, if any, from the larger turbocharger, I reviewed dyno charts from a number of S4’s equipped with TiAL 605 turbochargers and BorgWarner K04 turbochargers.

For this comparison I noted the peak torque reading and the torque value at 6500 rpm.  The difference was then calculated.  The result is a percentage change expressed as a negative value, which I made a positive number for ease of conversion into a boxplot chart.

The summary statistics are contained in the table below:

K04 and 605 Statistics
K04 and 605 Statistics

When put into a boxplot the results are:

K04 vs 605 Torque Dropoff
K04 vs 605 Torque Dropoff

The chart confirms that the larger 605 turbochargers do not suffer as much torque loss from peak as the engine speed increases to 6500 rpm.

The median torque loss for the 605’s is approximately 21% while the torque loss for the K04’s is approximately 26%.

I suspect the difference is not too substantial due to the source of the torque loss.  While the larger turbocharger is able to supply more airflow at higher engine speed, I believe the drop in engine volumetric efficiency as engine speed increases is causing the torque dropoff.